1. Field of the Invention
The present invention relates to an image forming apparatus which can form an electrostatic latent image without performing a precharging process, and can develop the image with developer into a visible image on a hard copy.
2. Description of the Related Art
Image forming apparatus which perform electronic photographing operate at high speed and low running cost and form high-quality images. For these advantages, they are used widely.
In most image forming apparatus performing electronic photographing, a corona charger pre-heats the photosensitive member, electrically charging the surface of the photosensitive member. Then, the light reflected from an original image is applied to the photosensitive member. The photosensitive member is discharged at those surface regions exposed to the light. The other surface regions which remain electrically charged define an electric charge image. Toner is applied to the surface of the photosensitive member and is to the electrically charged regions only, forming a visible image. The visible image is transferred onto a recording medium and subsequently fixed thereon, thus forming a hard copy of the original image.
Recently, copying machines and page printers, which perform electronic photographing, have been used in increasing numbers. They generate ozone while the photosensitive member is being pre-heated. If ozone leaks from the machines or printers, it is harmful to persons who are using them. Strict rules and regulations on ozone leakage have been come into effect in order to protect the users of copying machines and page printers. It is therefore strongly demanded that an image forming apparatus be developed which generates no ozone.
To meet this demand, an image forming method has been proposed in, for example, Jpn. Pat. Appln. KOKAI Publication No. 62-127853. In this method, an electric charge image is formed without electrically charging the photosensitive member before the member is exposed to the light reflected from an original image. More specifically, a pixel-electrode plate 1 shown in FIG. 1 is used in place of a photosensitive member. The plate 1 comprises a substrate 2, pixel electrodes 3 formed on the substrate 2 and arranged in rows and columns, and field-effect transistors (FETs) 4, each connected to one pixel electrode 3.
Each of the FETs 4 comprises a gate electrode 5 formed on the substrate 2, a gate insulating film 6 coated on the substrate 2 and the gate electrode 5, a semiconductor layer 7 formed on the film 6 and aligned with the gate electrode 5, a source electrode 8 connected to one end of the layer 7, and a drain electrode 9 connected to the other end of the layer 7. The drain electrode 9 is connected to the pixel electrode 3 associated with the FET 4, which is formed on the gate insulating film 6. The source electrode 8 is connected to the ground. The gate electrode 5 is connected to a driver circuit, which generates gate signals in accordance with image signals.
The pixel-electrode plate 1 is used to form a visible image identical to an original image, in the following way.
All pixel electrodes 3 are connected to ground, while all FETs 4 remain in operative state. Then, a bias voltage is applied to the pixel electrodes 3 by using a magnetic brush. Toner is applied to the plate 1, forming a thin layer having a uniform thickness on each pixel electrode 3. Developing electrodes, each having an insulated surface, are placed close to the pixel electrodes 3. A developing voltage is applied from the developing electrodes to the pixel electrodes 3. In this condition, gate signals are supplied to the gate electrodes 5 of the FETs 4 which have been selected in accordance with the image signals. The selected FETs 4 are thereby turned on, while the unselected FETs 4 remain off. An electric field is generated between the pixel electrode 3 connected to any selected FET 4 and the developing electrode facing this pixel electrode 3. This is because the pixel electrode 3 is connected to the ground. The pixel electrode 3 applies charge to the toner particles on it. The charged toner particles fly onto the developing electrode. Meanwhile, no magnetic field is generated between the pixel electrode 3 connected to any unselected FET 4 and the developing electrode facing this pixel electrode 3, and no toner particles are attracted to this pixel electrode 3. As a result, a toner image, which is visible, is formed on the developing electrodes.
The image forming method described above includes no pre-heating step and can form images on recording media, without generating ozone. In the method, however, a toner image must be formed within the operating time of the selected FETs 4. The operating time of each selected FET 4 is very short since all selected FETs 4 must be turned on within a short period, one after another. The higher the image-forming speed, the shorter the operating time of each FET 4. Each selected FET may be no longer operating before a sufficient amount of toner moves to the developing electrode, even if the magnetic field generating region is relatively large.
To make matters worse, the developing electrodes may fail to hold toner particles firmly since the associated pixel electrodes 3 come to have no electric potential once the FET 4 connected to the electrode 3 is turned off. Consequently, the toner image may be unstable.
Furthermore, the image forming method may increase the running cost of any apparatus employing the method. As is known in the art, the total area of the toner-holding developing electrodes is only 10% of the total area of all developing electrodes in the case of forming a page of document, and at most 50% of the total area of all developing electrodes in the case of forming a drawing. Hence, only a small part of the toner applied is actually used to form an image. The greater part of the toner needs to be recollected and re-used. In practice, however, a portion of the toner not used cannot be recollected and wasted, increasing the toner consumption and ultimately increasing the running cost of the apparatus performing this image forming method.